LED back light sources using a reflection plate have been widely used as a surface illuminant for compact display used for, for example, cellular phones.
FIG. 6 is an exploded perspective view showing a prior art surface illuminant which is exploded. In the drawing, a reference numeral 101 denotes a light source comprising, for example, LEDs or CFL (cold fluorescent tubes); 102 denotes a light guide plate which is adjacent to said light source. The light guide plate 102 is formed of a white and transparent plate made of polycarbonate or acrylic resin, etc. The light guide plate 102 has a flat face from which light is emitted and a reverse face on which a multiplicity of prism-like grooves in which the distance between the grooves becomes narrower as it is more distant from the light source are formed as shown in the drawing by etching or sand blast process, so that the light from the light source will be reflected on the faces of the grooves for uniformly emitting light from the entire surface of the light guide plate 102. A reference 103 denotes a white reflective film made of an appropriate synthetic resin such as polyester (PET), polycarbonate, which is bonded to the edge of the reverse side of the light guide plate 102 with a both-sided adhesive tape. The film 103 reflects the light which has been transmitted through the faces of the grooves having prism-like cross section for guiding the light to the light guide plate 102 again so that it is emitted externally from the surface of the light guide plate 102.
In the above-mentioned arrangement, when the light source 101 is lit, the light from the light source 101 is transmitted through the light guide plate 102 and is then reflected on the prism-like faces formed on its reverse side or alternatively it is transmitted through the prism-like face and is reflected on the reflective film 103, and is incident upon the light guide plate 102, where it is mixed with the reflected light for substantially uniformly illuminating the entire surface of the light guide plate 102.
At this time, the light from the light source is emitted as back light having unchanged color since the light guide plate is colorless and transparent.
It has been known that a problem occurs in which a surface illuminant having white and intermediate color can not be obtained since LED is not capable of emitting white or intermediate color light when LEDs are used as light source.
(1) In order to overcome this problem, JA-P-7-176794 proposes a surface illuminant comprising a fluorescent light scattering layer made of a mixture of a fluorescent material with white powder for scattering fluorescent light, which are applied on either one of the sides of a light guide plate in which light emitted from blue light LEDs is subjected to wave-length conversion by said fluorescent light scattering layer and a desired color including white color can be provided by changing the fluorescent material.
(2) Alternatively, JP-A-8-7614 proposes a surface illuminant comprising a scattering layer which is formed by applying white powders for scattering fluorescent light on either one of the sides of a light guide plate and further comprising a transparent film containing a fluorescent material, which is disposed on the main face of the light guide plate opposite to said scattering layer, whereby desired colors including white color can be provided.
However, it is inconvenient that the fluorescent scattering layer should be applied (printed) on the light guide plate since the fluorescent material is directly applied upon the light guide plate in the surface illuminant as set forth in paragraph (1). Since the color tone may change depending upon the mixture ratio between the fluorescent material and white powder for scattering fluorescent light, a desired color can not be readily obtained.
In the surface illuminant as set forth in paragraph (2), it is also necessary to apply the fluorescent material and to provide a transparent film on which the fluorescent material is to be applied on the surface of the light guide plate in addition to the reflective plate provided on the reverse side of the light guide plate. This type of surface illuminant is complicated in structure and its assembly requires a long time and much labor.
Since the light in the light guide plate is terminated unless the light source is continuously lit in the above-mentioned both surface illuminants, it is necessary to constantly turn on the light source when the surface illuminant is used. This problem is not negligible in the portable electronic devices such as cellular phones from the view point of power consumption since all the power is supplied from a battery cell.